Influenza A virus causes one of the most widespread infections in humans. In a typical year, it infects 10-20% of the population in the U.S., causing up to 40,000 deaths. During the 1918 influenza virus pandemic, over 20 million people died worldwide. The threat of another pandemic continues even now because, despite intensive efforts, there is still no effective therapy for influenza infection and prevention is haphazard. Existing vaccines are of limited value because the influenza viruses they target are determined by "best guess," based upon recently prevalent strains. The virulence of influenza A virus results from i) its easy spread by aerosol, ii) its ability to escape from protective immunity, and iii) the periodic emergence of new, virulent strains of the virus. For these reasons, the National Institutes of Health has designated influenza infection as a top priority area for biodefense. RNA interference (RNAi) is a process by which double-stranded RNA directs sequence-specific degradation of messenger RNA in animal and plant cells. Studies have shown that RNAi can be triggered by synthetic 21-nucleotide duplexes of small interfering RNA (siRNA) molecules. This project proposes to investigate siRNAs in both preventing and treating influenza virus infection. Specifically, we propose to i) identify siRNAs that potently inhibit influenza virus production in cultured cells, ii) develop systems for efficient delivery of siRNAs into cells of the upper respiratory tract and the lung, where influenza infection normally occurs, iii) investigate the in vivo efficacy of siRNA inhibition of influenza infection in mice, and iv) elucidate the mechanisms by which siRNAs inhibit influenza virus production. Results obtained from the proposed studies may provide a basis for further development of siRNAs as prophylaxis and therapy of influenza virus infection in humans. Theoretically, an siRNA-based treatment should remain effective even with the emergence of new strains of the virus.